linux-kernel - Re: ARM seccomp filters and EABI/OABI

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Date:	Mon, 28 Oct 2013 17:53:26 -0400
From:	Paul Moore <paul@...l-moore.com>
To:	Richard Weinberger <richard.weinberger@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	libseccomp-discuss@...ts.sourceforge.net,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ARM seccomp filters and EABI/OABI

On Thursday, October 24, 2013 09:55:57 PM Richard Weinberger wrote:
> On Wed, Oct 23, 2013 at 11:02 PM, Andy Lutomirski <luto@...capital.net> 
wrote:
> > I'm looking at the seccomp code, the ARM entry code, and the
> > syscall(2) manpage, and I'm a bit lost.  (The fact that I don't really
> > speak ARM assembly doesn't help.)  My basic question is: what happens
> > if an OABI syscall happens?
> > 
> > AFAICS, the syscall arguments for EABI are r0..r5, although their
> > ordering is a bit odd*.  For OABI, r6 seems to play some role, but I'm
> > lost as to what it is.  The seccomp_bpf_load function won't load r6,
> > so there had better not be anything useful in there...  (Also, struct
> > seccomp_data will have issues with a seventh "argument".)
> > 
> > But what happens to the syscall number?  For an EABI syscall, it's in
> > r7.  For an OABI syscall, it's in the swi instruction and gets copied
> > to r7 on entry.  If a debugger changes r7, presumably the syscall
> > number changes.
> > 
> > Oddly, there are two different syscall tables.  The major differences
> > seem to be that some of the OABI entries have their argument order
> > changed.  But there's also a magic constant 0x900000 added to the
> > syscall number somewhere -- is it reflected in _sigsys._syscall?  Is
> > it reflected in ucontext's r7?
> > 
> > I'm a bit surprised to see that both the EABI and OABI ABIs show up as
> > AUDIT_ARCH_ARM.
> > 
> > Can any of you shed some light on this?  I don't have an ARM system I
> > can test on, but if one of you can point me at a decent QEMU image, I
> > can play around.
> 
> Maybe this helps:
> http://people.debian.org/~aurel32/qemu/armel/

Thanks for the pointer, although those images look quite old, has anyone done 
a refresh?

Also, on a related note, does anyone have any experience with any of the cheap 
PC-esque ARM boards/systems that are floating around?  I'm to the point of 
considering picking one up for libseccomp development if I can find one that 
supports a standard development environment, decently responsive, and is 
relatively cheap ... anyone?

-- 
paul moore
www.paul-moore.com

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